Anaerobic microorganisms can produce ethanol from carbon monoxide (CO) through fermentation of gaseous substrates. Fermentations using anaerobic microorganisms from the genus Clostridium produce ethanol and other useful products. For example, U.S. Pat. No. 5,173,429 describes Clostridium ljungdahlii ATCC No. 49587, an anaerobic microorganism that produces ethanol and acetate from synthesis gas. U.S. Pat. No. 5,807,722 describes a method and apparatus for converting waste gases into organic acids and alcohols using Clostridium ljungdahlii ATCC No. 55380. U.S. Pat. No. 6,136,577 describes a method and apparatus for converting waste gases into ethanol using Clostridium ljungdahlii ATCC No. 55988 and 55989.
The CO is often provided to the fermentation as part of a gaseous substrate in the form of a syngas. Gasification of carbonaceous materials to produce producer gas or synthesis gas or syngas that includes carbon monoxide and hydrogen is well known in the art. Typically, such a gasification process involves a partial oxidation or starved-air oxidation of carbonaceous material in which a sub-stoichiometric amount of oxygen is supplied to the gasification process to promote production of carbon monoxide as described in WO 2009/154788.
Fermentation of gaseous substrates can be challenging because at least a portion of the gaseous substrate must dissolve in an aqueous fermentation broth before the substrate can be metabolized by the microbial culture. Fermentations where the gaseous substrate provides the carbon and energy source for the microorganism are particularly challenging due to the large amount of substrate needed to be solubilized in the fermentation broth before metabolism can take place. Substrates such as CO which have a low solubility in an aqueous fermentation broth require a highly efficient mass transfer into an aqueous fermentation broth as the CO provides a carbon source for the anaerobic fermentation. Attempts to improve CO mass transfer are described in U.S. Pat. Nos. 5,972,661 and 7,201,884 and in WO 2011/028137.
Fermentation processes with acetogenic bacteria typically include one or more seed reactors, one or more growth reactors and at least one main reactor. Acetogenic bacteria are normally grown to a certain cell density in a seed reactor. The seed reactor is then used to inoculate a growth fermentor. The growth fermentor will usually be of a larger size than seed reactor. Acetogenic bacteria in the growth reactor are then grown to a desired cell density. The growth reactor may then be used to inoculate another larger growth reactor or may be used to inoculate a main reactor. The main reactor will be of a larger size than the growth reactor. The use of multiple reactors increases start-up times and increases costs.